1. Field of the Invention
The present invention relates generally to radar augmentation systems for use with airborne targets. More particularly, the present invention relates to a system for maximizing the usable gain of the augmentation system of an airborne target and thus its effective echo strength by increasing isolation in the target's radar augmentation system.
2. Description of the Prior Art
Airborne targets are in wide use for evaluation of missile performance and for training of firing crews. Since the objective is to hit the target, more direct hits mean greater target losses. In order to decrease these losses, unpowered targets have been towed behind a piloted aircraft or a drone aircraft, and inexpensive powered targets have been launched from a carrier aircraft. Also, targets are built to be recovered and re-used.
In the case of targets for missiles guided by radar, the target may be made with minimum size and construction cost, and be equipped with radar augmentation which produces the desired radar return signal to simulate a large target, thus allowing the missile guidance system track on the target.
There are both passive and active radar augmentation systems. A passive augmentation system may be, for example, a Lundberg lens which is commonly used in the nose of a target to reflect an augmented radar return signal.
Active radar augmentation systems generally consist of linear repeaters which receive, amplify, and re-radiate a captured signal back to the radar emitting the signal to increase the apparent strength of the radar echo. The radar observing the target carrying the augmentation system perceives a radar echo significantly stronger than the target echo, thus emulating a larger target.
An example of an active radar augmentation system may be found in U.S. Pat. No. 4,178,596 to Robert P. Rowlett. The system disclosed in U.S. Pat. No. 4,178,596 comprises a transmitting antenna located at the rear of a target which has a wide angle forward directional pattern. There is also a receiving antenna located in the forward portion of the target and an amplifier which is connected between the antennas.
It is desirable to have high gain RF amplifiers in active radar augmentation system to produce the desired radar return signal making a missile's guidance system track the target. While RF amplifiers with high gain are commercially available, the usable gain is limited by the isolation between transmitting and receiving antennas, which are typically in close proximity due to the physical constraints of the target. The coupling between antennas provides a feedback path whereby the signal transmitted by the augmentation system is captured by the augmentation system's receiving antenna, amplified and re-radiated, along with the desired radar signal component. The undesired signal resulting from the finite antenna isolation recirculates through the system and will generally result in regenerative oscillations which can negate the effectiveness of the radar augmentation system.
The limited gain allowable in compact augmentation systems having intrinsically low antenna isolation limits the maximum radar cross section that can be achieved. A number of techniques using cross-polarized antennas, baffles of metal and radar absorbing materials interposed between receiving and transmitting antennas have met with minor success. In the prior art, radar cross section values in excess of 10 m.sup.2 have been difficult to achieve as a result of the limited degree of antenna isolation. It is desired, however, that a compact self-contained augmentation system be capable of providing 100 m.sup.2 radar cross section at X-band frequencies for airborne and seaborne targets.
Accordingly, there is a need to increase antenna isolation in active radar augmentation systems to prevent regenerative oscillations within the system and provide adequate radar cross section for airborne and seaborne targets.